This invention relates to fluid-pressure-operated percussion drills of the kind known as "down-the-hole" drills, in which a tubular drill body containing a percussive hammer piston and the cylinder in which it reciprocates is passed down the hole being drilled together with the drilling bit carried by the drill body, instead of remaining above ground level. The rotation of the drill bit is obtained by rotating the whole drill, on the end of the string of tubes on which the drill body is mounted, by means of a rotary motor which remains outside the hole.
In down-the-hole drills it is usual to deliver the used working fluid exhausted from the working chambers above and below the piston to the drill bit, where it passes around the cutting edges of the bit so as to flush away rock chippings. Thus it is necessary to provide passages in the drill body to carry the pressure fluid, usually compressed air, past the hammer piston to the lower working chamber for lifting the piston, and to carry the exhaust fluid from the upper and lower chambers to the drill bit for flushing. To avoid increasing the diameter of the drill body for this purpose, it has been proposed to provide a central tube extending longitudinally through a central axial bore in the hammer piston, which slides up and down this tube in the cylinder when reciprocating, the tube carrying fluid down past the piston for exhaustion to the bit.
An object of the present invention is to provide a down-the-hole drill having a central axial tubular structure of simple construction which is utilised to carry all the pressure fluid required below the hammer piston, thereby obviating the need for any further passages either in the drill body or in the hammer for this purpose.